A growing body of evidence indicates that mechanical cues modulate the development and repair of skeletal tissues by regulating gene expression and tissue differentiation. In particular, several studies have shown that these cues play a crucial role during the bone repair process which involves mechano-sensitive, pluripotent mesenchymal cells. These results suggest that a more comprehensive characterization of cellular responses to specific mechanical stimuli will identify underlying mechanisms that regulate the fate of mesenchymal tissue. As such, the overall goal of this work was to characterize the molecular expression and tissue phenotypes induced by a controlled bending motion applied to a healing osteotomy gap.

A previously established in vivo rat model of controlled mechanical stimulation was used to carry out three specific aims. First, the distribution of tissues created within and surrounding the osteotomy gap were characterized using standard histology and new 3D rendering techniques. Second, the temporal and spatial molecular profiles were quantified for both a small set of candidate genes known to be involved in bone and cartilage formation as well as a larger set of genes implicated in skeletal development, skeletal repair, and mechanotransduction. Third, the distribution of strains created throughout the osteotomy gap was quantified in order to compare these distributions to those of the tissue and molecular phenotypes.

Bending stimulation substantially altered the bone healing response at both tissue and molecular levels. Stimulated specimens exhibited an increase over time in the amount of cartilage formed, whereas control specimens demonstrated partial bony bridging. Bending stimulation induced up-regulation of cartilage-specific genes, and by the end of the stimulation timecourse, the glycosaminoglycan content of the stimulated callus tissue was no different from that of native articular cartilage. Examination of the strain distributions revealed a correlation between tensile strain magnitudes and the type of tissue formed. These results together demonstrated that the applied mechanical stimulus altered the normal trajectory of bone healing, resulting in differential molecular expression and the formation of cartilaginous tissues with similar composition and structure to those of articular cartilage. These findings illustrate how mechanical cues can direct tissue differentiation during skeletal healing, and they further indicate the possibility of using mechanical stimuli to elucidate alternative treatment regimens for injured or diseased diarthroidal joints.